Automatic blood pressure monitors are commonly used to periodically measure the blood pressure of a patient. In most automatic blood pressure monitors, a pressure cuff is attached to a patient's arm over the brachial artery. The cuff is first pressurized with an applied pressure that is high enough to substantially occlude the artery. The cuff pressure is then gradually reduced, either continuously or in increments. As the pressure is reduced to systolic pressure, the flow of blood through the brachial artery beneath the cuff increases substantially.
When the blood flows through the brachial artery following each contraction of the heart, it imparts a pulsatile movement to the wall of the artery. This pulsatile movement is coupled to a blood pressure cuff extending over the artery as minute changes in cuff pressure, which are known as oscillometric pulses. Automatic blood pressure monitors can use these oscillometric pulses to indicate a patient's blood pressure. Conventional automatic blood pressure monitors measure and record the amplitude of the oscillometric pulses at a large number of discrete cuff pressures. After the blood pressure measurement has been completed, a recorded table contains the oscillometric pulse amplitude at each cuff pressure.
In theory, the systolic, diastolic and mean blood pressure can then be determined from the values in the table using empirical definitions of these parameters as a function of the amplitudes of these oscillometric pulses. However, blood pressure measurements are often adversely affected by artifact, generally produced by patient movement. Motion-induced artifact can substantially alter the measured amplitude of oscillometric pulses thus introducing inaccuracies in the measurement of oscillometric pulse amplitudes.
Techniques have been developed to determine the systolic and diastolic pressures even in the presence of artifact. However, it is substantially more difficult to determine the mean arterial pressure in the presence of artifact. The mean blood pressure occurs at the point where the oscillometric pulse has the largest amplitude. As mentioned above, artifact induced noise often obscures the largest oscillometric peaks by either making the peak oscillometric pulse appear to be smaller or by making other oscillometric pulses appear to be larger. It is thus often not possible to determine the peak oscillometric pulse by direct measurement. As a result, while the systolic and diastolic pressures can usually be accurately obtained using automatic blood pressure monitors, it is often not possible to obtain accurate measurements of mean arterial pressure using conventional automatic blood pressure monitors.